This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power for vehicle propulsion.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor, also known as a reformer. The reformer contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas including primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a reformer which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The reformer produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors such as a water/gas shift (WGS) and preferential oxidizer (PROX) reactors are used to produce carbon dioxide (CO2) from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,422 and 08/980,087, filed in November, 1997, and 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively control the quality of the hydrogen feed stream to the fuel cell, especially maintaining a low level of carbon monoxide within the hydrogen gas stream supplied to the anode in the fuel cell stack. This is particularly difficult during transient operation of a vehicular fuel cell system wherein the reformate fuel requirement varies with the changing loads placed on the fuel cell.
Therefore, it is desirable to provide a method to control the amount of carbon monoxide in the hydrogen rich gas stream to the anode and at the same time minimizing the consumption of the hydrogen in the anode fuel stream.
The present invention is directed towards controlling the supply of oxygen for the promotion of oxidization of carbon monoxide while avoiding excessive oxidization of hydrogen in reformate stream useable in a fuel cell system.
The fuel cell system of the present invention comprises a source of a reformate stream which contains hydrogen and carbon monoxide (CO). The reformate stream typically contains more hydrogen on a volume basis than CO. The fuel cell system further comprises a PROX reactor for selectively oxidizing the CO through contact of the stream with a catalyst inside the PROX reactor chamber. The catalyst is supported by a carrier within the chamber and the chamber includes an inlet and an outlet allowing the stream to pass through the reactor chamber and over the catalyst.
The fuel cell system further comprises an apparatus for supplying controlled amounts of an oxidant, gaseous mixture containing oxygen, or preferably air, into the stream containing hydrogen and CO at a predetermined location or locations along the stream. This control is preferably achieved by an air injector. In one location, the injector is placed downstream from the reformate source, but upstream from the PROX reactor. Another location is directly into the PROX reactor.
A precisely controlled supply of an oxidant provides a sufficient amount of oxygen to promote oxidation and thereby consumption of CO with minimal or lesser oxidation and consumption of hydrogen in the stream which is essential for efficient operation of the fuel cell and system. The advantageous result is a CO depleted reformate stream with high levels of hydrogen feeding the anode in the fuel cell. The fuel cell system further comprises one or more fuel cells downstream of the PROX reactor which receives and consumes the hydrogen-rich, CO depleted reformate stream to produce electrical energy.
The preferred oxidant is air. Other oxidants include oxygen-containing compounds, and mixtures comprising oxygen, and essentially pure oxygen. For convenience, the invention will be further described with reference to the preferred oxidant which is air.
The present invention may also be used to control injection of air between the PROX reactor and the fuel cell. This promotes oxidation of remaining unreacted CO in the stream prior to the stream entering the anode inlet in the fuel cell. Thereby, the CO content of the stream is further reduced.
In its broadest aspect, the invention may be utilized in any system having a first vessel upstream of a second vessel wherein the second vessel is used to oxidize a constituent of the gas stream supplied from the first vessel. The method includes measuring the flow rate of inlet streams to the first vessel; determining the flow rate of the outlet stream from the first vessel, and determining the composition of the outlet stream; determining the flow rate of oxidant injection to the second vessel; measuring the pressure of the first vessel; and adjusting the flow rate of oxidant injection to the second vessel. As an alternative, the pressure of the first vessel can be measured in the outlet stream of the first vessel.
An aspect of the invention is the computation of the oxidant injection. The oxidant injection is computed from the requirements based upon the inlet streams to the first vessel. The oxidant requirements are subsequently modified and updated on a transient basis based upon changes in the pressure within the first vessel. The oxidant requirements follow the formula, FOn=OFSxe2x88x92k*(Pnxe2x88x921/Pn)*(OFSxe2x88x92FOnxe2x88x921), wherein OFS is the oxidant computed from the feed streams to the first vessel, Pn is the pressure of the first vessel at time Tn, Pnxe2x88x921 is the pressure of the first vessel at an earlier time Tnxe2x88x921, FOnxe2x88x921 is the flow rate of oxidant at the earlier time Tnxe2x88x921, k is a lag filter coefficient, and FOn is the computed flow rate of oxidants.
Another embodiment of this invention is the application of the invention to the preferential oxidizer and the fuel cell. The oxidant injection to the preferential oxidizer in the fuel cells is controlled by determining flow rates of the inlet streams to the reformer upstream of the preferential oxidizer; determining the flow rate of the outlet stream and the composition of the outlet stream, more particularly the carbon monoxide content; determining the pressure in the reformer; determining the oxidant injection to the preferential oxidizer; and using a different lag filter coefficient, determining the oxidant injection to the fuel cell.
In another aspect, the oxidant injection is adjusted to the preferential oxidizer and the fuel cell. The oxidant injection to the preferential oxidizer is controlled by determining the flow rates of the inlet streams to the reformer; determining flow rate of the outlet stream from the reformer; determining the carbon monoxide content in the outlet stream from the reformer; determining the pressure in the reformer; and determining the oxidant injection to the preferential oxidizer. The oxidant injection to the fuel cell is controlled by determining the flow rates of the inlet streams to the preferential oxidizer; determining the flow rate of the outlet stream from the preferential oxidizer; determining the carbon monoxide content in the outlet stream from the preferential oxidizer; determining the pressure in the preferential oxidizer; and determining the oxidant injection to the fuel cell.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.